Detection and identification or at least classification of unknown substances has long been of great interest and has taken on even greater significance in recent years. Among advanced methodologies that hold a promise for precision detection and identification are various forms of spectroscopy, especially those that employ Raman scattering. Spectroscopy may be used to analyze, characterize and even identify a substance or material using one or both of an absorption spectrum and an emission spectrum that results when the material is illuminated by a form of electromagnetic radiation (e.g., visible light). The absorption and emission spectra produced by illuminating the material determine a spectral ‘fingerprint’ of the material. In general, the spectral fingerprint is characteristic of the particular material or its constituent elements facilitating identification of the material. Among the most powerful of optical emission spectroscopy techniques are those based on Raman-scattering.
Raman-scattering optical spectroscopy employs an emission spectrum or spectral components thereof produced by inelastic scattering of photons by an internal structure of the material being illuminated. These spectral components contained in a response signal (e.g., a Raman signal) may facilitate determination of the material characteristics of an analyte species including identification of the analyte.
Unfortunately, the Raman signal produced by Raman-scattering is extremely weak in many instances compared to elastic or Rayleigh scattering from an analyte species. The Raman signal level or strength may be significantly enhanced by using a Raman-active material (e.g., Raman-active surface), however. For example, a surface that includes a Raman-active material may be employed in surface enhanced Raman-scattering (SERS) optical spectroscopy to significantly enhance a signal level or intensity of the Raman signal produced by a particular analyte species. While SERS has proven to yield good results in many applications, further improvements are still being sought.
For example, SERS often suffers from or exhibits unpredictable hot spots across the surface. The hot spots produce much higher-level Raman signals than surrounding areas but the location and quantity of these hot spots can be difficult to control. As such, it is often necessary to flood the entire surface with analyte to insure that sufficient analyte reaches the hot spots and produces a detectable Raman signal. Requiring the surface to be flooded precludes detection of very small amounts of analyte (e.g., single molecules) and also hinders identifying other analyte characteristics such as species distribution within a sample.
Attempts to localize or control the production of hot spots have included the use of sharp tips in conjunction with a SERS surface in what is known as tip enhanced Raman spectroscopy (TERS). In TERS, a sharp, conductive tip is placed very close to but spaced apart from the SERS surface. The tip acts as an antenna concentrating and locally enhancing the electromagnetic field in a region between the tip and the surface. While producing results including detection of extremely small quantities of analyte, TERS presents many practical challenges to implementation and use.
Certain embodiments of the present invention have other features that are one of in addition to and in lieu of the features illustrated in the above-referenced figures. These and other features of the invention are detailed below with reference to the preceding drawings.